Semiconductor devices are typically manufactured by applying layers to semiconductor wafers through a series of deposition and etching steps. The processes appropriate for application of a particular layer often are dictated by, and compatible with, the layers that have been previously applied. At points during the semiconductor manufacturing process, a wafer may have a surface formed of a low dielectric constant dielectric, for example, through which have been etched vias extending through to an underlying conductive layer. These vias or other features will be distributed over the surface of the wafer in varying densities, with areas of dense patterned features and areas of no features or of isolated patterned features or no features. Over such a surface, a dielectric layer of bottom-anti-reflective-material type is commonly applied by a spin-on process. Such layers are referred to as spin-on-glass (SOG) or spin-on-dielectric layers.
Spin-on-dielectric layers are applied by rotating an upwardly facing patterned wafer on a spinning chuck and dispensing the dielectric coating material in liquid form onto the spinning wafer, where it flows from the center toward the edge by centrifugal force, coating the wafer. The liquid material contains a solvent that ultimately evaporates, setting the dielectric layer as a solid film on the substrate. While the objective is to spin the wafer to distribute the film to a uniform thickness by liquid convection, at some point evaporation becomes the dominant mechanism in determining the final dry film thickness profile.
It is found that the final dry film thickness in SOG films varies over the surface of a patterned wafer to which the film is applied. The density of the patterned features on the wafer surface have been found to affect the thickness of the film. In areas where no, or only isolated, features are present on the surface, the film is found to be substantially thicker than in areas where the features are denser. As a result, in a subsequent etch step, patterns that are applied are etched deeper than necessary into the underlying low-constant dielectric in areas of high feature density where the SOG film is thin in order to achieve the etching depth in areas of low feature density where the SOG film is thick. This is a disadvantage. Ideally, the SOG layer should be uniform.
For example, the dynamics of centrifugal force is used to uniformly disperse liquid material over a wafer surface. The wafer may be spinning when the SOG liquid containing the volatile solvent is dispensed (dynamic dispense), or the wafer may be stationary during the dispensing of the liquid (static dispense). In either event, after the liquid is dispensed, the wafer is spun to spread the coating by the mass transfer associated with centrifugal force, liquid convection. During spinning, the casting solvent is rapidly evaporating and the film viscosity is rapidly changing. Evaporation eventually changes the viscosity of the film enough to slow or stop the mass transfer and the film profile is set. Further casting solvent evaporation sets final coat film thickness.
Using a traditional spin coating process with either static or dynamic dispensing of the liquid onto the wafer, spin-on-glass or spin-on-dielectric applications leads to a difference in thickness between areas on the wafer having no patterned features or having only isolated patterned features compared to areas of dense patterned features. In a via-first architecture, this thickness difference, referred to as the iso-dense bias, leads to a decrease resist focus margin, especially during etching with Argon Fluoride (ArF) processes with non-uniform trench heights.
Reduction or elimination of iso-dense bias between areas on the wafer surface of no patterned features or isolated patterned features (blanket thickness) compared to the thickness on areas of dense patterned features would provide more focus process margin during photolithography processes and greater trench depth control during etching processes.
In the conventional single-coat spin-on process discussed above, iso-dense bias is inherent. The only variable historically found to improve iso-dense bias was adjusting solid contents of the spin-on-dielectric or spin-on-glass material. Double-coat spin-on processes have been employed which attempt to use two different chemical formulations with a bake step in between to control iso-dense bias. With the first coat of the double-coat process, gap features are partially filled leaving minimal material in isolated areas (blanket thickness). The second coat, which has flexibility in its solids content, fills the remainder of gap features and defines the blanket thickness. The double-coat process must balance the partial fill ratio of the first coat process with field thickness deposited with the first coat to be successful. The double-coat process is supposed to be capable of better results than single-coat process, but it is more complicated, slower and more costly.
In a process referred to as the scan-coater spin-on process, which is described in U.S. Pat. No. 6,800,569, droplets of chemical are dispensed through a discharge nozzle onto the wafer repetitively, while scanning across the wafer in the x and y directions. Then, the process uses a low pressure cooling dry chamber to evaporate solvent material. This process eliminates radial mass transport, but requires discharge sufficient to fill the most dense pattern areas on the wafer, with no control over iso-dense bias. Further, in a process referred to as the vapor solvent coating process, which is carried out in an SME-solvent mediated environment as described in U.S. Pat. No. 5,670,210, attempts are made to control the evaporation rate of the spun-on material by controlling the vapor pressure above the dispensed film. Its principle is that, by preventing some of the effects of evaporation, the duration of mass transport of the liquid coating by liquid convection diffusion can be extended. In theory, better control of field thickness by mass transport would result. This process controls the evaporation portion of the process, but does not alter whatever iso-dense bias results from the mass transport portion of the process.
Other ways to handle iso-dense bias focus on the addition of subsequent process steps. For example, chemical-mechanical polish has been proposed to level the spun-on film. Additionally, wet-etch back process steps have been added for the same purpose. The addition of extra steps is inherently undesirable as they decrease productivity. Further, each process has the potential of adding defect generation and can be destructive to underlying materials previously deposited on the wafer in the semiconductor manufacturing process.
Accordingly, there is a need for a better and simpler way to eliminate iso-dense bias in spin-on-dielectric processes.